So if someone posts a message, and someone responds to it, we call that a 'reply'.

However, I am struggling to think of a good phrase for the situation where you have the reply, and you want to see what it is a reply to? What would we call the entity that it is a reply to? "See message" doesn't seem to semantically capture it.

"Original post" would be a natural choice in the context of IT. However, I think if the reply has a nature of question-answer, I think inquiry should sound better. And, even when the nature of question-answer is not obvious, a simple word such as subject could also work.
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Damkerng T.May 23 '14 at 21:08

I agree that in "techie" contexts, parent message is the preferred term. But for everyday use, original message is good where more specific words like query or request aren't suitable for some reason. And of course, in forums such as this we often use OP for Original Post or Original Poster.
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FumbleFingersMar 28 '11 at 15:20

@FumbleFingers: But OP typically refers to the original post of a thread. If W. Quine starts the thread, you post to the thread, and I reply to you, neither you nor your message is the OP.
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DrewMay 23 '14 at 15:41

@Drew: I don't know about other SO sites (or "typical" usage elsewhere on the Net at large), but I'd say the vast majority of uses of OP on ELU refer to Original Poster. Of course, an awful lot of them could be interpreted either way ("the OP says blah blah"), but many are unambiguous ("OP thinks blah blah"). So perhaps if you started with the preconception OP=Original Post you'd see more support for that interpretation than is actually justified.
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FumbleFingersMay 23 '14 at 17:20

@FumbleFingers: Whether you interpret OP as original post or original poster, my point is that it typically refers to the start of the thread, not to the post or poster to which you are replying. I'm just saying that OP is not the answer to the question, IMO.
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DrewMay 23 '14 at 19:01

If there is a single message and a single response, then the first is referred to as the original message.

If this a mailing list discussion or forum discussion, then the first message can be referred to as the original post or OP (OP can also refer to person who submitted the first post: the original poster). The collection of original post and responses are referred to as a thread. If you were unable to see the messages or posts before a certain response, you might say you didn't see the original post or you couldn't see the entire thread.

You might use the word prompt in either its noun or verb form, as in, "What was the prompt for this reply?" or "What prompted this reply?" This would be--I think--a non-technical, not jargonized, word for the concept you are trying to articulate.

This is because reply is not a noun with a specific and direct relative. It is a very general word.

Here are some nouns and their counterparts:

Parent - Children

Question - Answer

Ancestor - Descendant

The difference between children, answer and descendant is that each of them has a direct relation to their precursor. To be a child can only be an offspring of a parent, an answer can only denote the existence of a question and a descendant will have to have ancestors. It's basically law.

Reply has no specific relative because an answer is also a reply but a reply may or may not be an answer!

"Reply" is very general and the only other general word I can think of that is most relevant is "message".

For and against argument:

A parent denotes a child and a child denotes a parent. However, a question does not denote an answer but an answer denotes a question.

Hence, the answer to your question will have to be based on "reply" in order to be 100% accurate with the other definitions of counterparts - the existent of A means also the existence of B but may not be vice versa, yet they are still considered counterparts.